In flat alkaline primary batteries such as coin-shaped batteries and button-shaped batteries used for compact electronics such as electronic wrist watches and portable electronic calculators, a positive electrode can contains a positive electrode mixture where manganese dioxide is used as a positive electrode active material and a negative electrode can contains a negative electrode mixture where zinc or zinc alloy powder is used as a negative electrode active material.
The positive electrode mixture and the negative electrode mixture face each other with a separator in between, and an alkaline electrolyte is injected inside the battery. The manganese dioxide in the positive electrode mixture, graphite powder, which is a conductive agent, and in the case where the binding properties of the positive electrode mixture are poor, a resin powder or emulsion such as polytetrafluoroethylene (PTFE), which is a binding agent, are mixed and then compressed into a pellet which is used as the positive electrode.
In addition, so-called silver oxide batteries where silver oxide is used as the positive electrode active material in flat alkaline primary batteries are widely available in the general market. Silver oxide has a high volume energy density compared to manganese dioxide, and the battery voltage where the positive electrode active material is silver oxide and the negative electrode active material is zinc is flat in the vicinity of 1.56 volts. Therefore, silver oxide is mainly used in power supplies for compact electronics such as electronic wrist watches and portable electronic calculators the final voltage of which is not lower than 1.2 volts.
Although silver oxide provides excellent performance, it is expensive because silver, which is a precious metal, is the main component and its price fluctuates depending on the market rate for silver. Thus, silver oxide is a difficult material for use in reducing or stabilizing the manufacturing cost. Therefore, a great number of so-called alkaline manganese batteries are spread in the general market as well as silver oxide batteries. The positive electrode active material in the alkaline manganese batteries is manganese dioxide, the price per mass of which is much less expensive, approximately one-two-hundredths of silver oxide, although the volume energy density of the alkaline manganese batteries is lower and the voltage drop due to discharge is greater than those of silver oxide.
In order to improve the above described performance of manganese dioxide, it has been studied that various additives are added to inexpensive active materials such as manganese dioxide (for example, see Japanese Unexamined Patent Publication No. 2003-234107, Japanese Unexamined Patent Publication No. 2004-6092, Japanese Unexamined Patent Publication No. 2005-19349).
Flat alkaline primary batteries where manganese dioxide is used as a positive electrode active material have a problem where the voltage drops greatly due to discharge. The voltage drop of manganese dioxide due to discharge causes the problem that the time of use in devices, such as electronic wrist watches where the final voltage is set to a high value for the voltage of silver oxide batteries, is extremely shortened. Although various studies have been conducted in order to prevent the voltage drop, satisfactory results have not been obtained.